1. Field of the Invention
The present invention relates to a liquid crystal display device for use in television sets, personal computers, word processors, office automation (OA) equipments or the like, and also relates to a method for driving the same.
2. Description of the Related Art
Since liquid crystal display devices are thin and light, they are used in, for example, television sets, personal computers, word processors and OA equipment. Many of such liquid crystal display devices utilize the fact that liquid crystal molecules have an anisotropy of refractive index and an anisotropy of dielectric constant. In such liquid crystal display devices, a voltage is applied across a liquid crystal layer, whereby optical modulation is conducted by an electric field produced by the voltage.
Since liquid crystal display devices are thin and light, they are used in, for example, television sets, personal computers, word processors and OA equipment. Many of such liquid crystal display devices utilize the fact that liquid crystal molecules have an anisotropy of refractive index and an anisotropy of dielectric constant. In such liquid crystal display devices, a voltage is applied across a liquid crystal layer, whereby optical modulation is conducted by an electric field produced by the voltage.
In such a liquid crystal display device, gate lines and source lines are arranged in a matrix, and a pixel electrode and a thin film transistor are formed at each of the regions surrounded by the gate lines and the source lines. Thus, a voltage across each pixel electrode is controlled by a corresponding thin film transistor. Such a voltage application method will be described later in detail.
Hereinafter, a conventional liquid crystal display device will be described.
FIG. 15 is a cross sectional view schematically showing the conventional liquid crystal display device. FIG. 16 is a plan view of a matrix substrate used in the conventional liquid crystal display device. FIG. 17 is a cross sectional view taken along line 17xe2x80x9417xe2x80x2 of FIG. 16.
As shown in FIG. 15, the liquid crystal display device includes a matrix substrate 41, a counter substrate 42 facing the matrix substrate 41, and a liquid crystal layer 43 interposed therebetween. The liquid crystal layer 43 is formed by introducing a liquid crystal material into the gap between the matrix substrate 41 and the counter substrate 42.
In the matrix substrate 41, source lines 44, gate lines 45, thin film transistors 46 and pixel electrodes 47 are formed on a transparent substrate, as shown in FIG. 16. The transparent substrate is formed from glass or the like. The source lines 44 and the gate lines 45 are arranged in a matrix. A voltage for each pixel electrode 47 is applied from a corresponding source line 44 via a corresponding thin film transistor 46.
In the counter substrate 42 (not shown in FIG. 16), a light-shielding film (not shown), a color filter (not shown) and a planar counter-electrode (not shown) are formed on a transparent substrate. The transparent substrate is formed from glass or the like. The light-shielding film has openings corresponding to the pixel electrodes 47.
FIG. 17 shows a cross-sectional structure of the thin film transistor 46. A semiconductor layer 50 is formed on a gate electrode 48 with an insulating film 49 interposed therebetween. The gate electrode 48 extends from a corresponding gate line 45, as shown in FIG. 16. A source electrode 51 and a drain electrode 52 are formed thereon so as to be spaced apart from each other. The source electrode 51 extends from a corresponding source line 44, and is electrically connected to the source line 44. The drain electrode 52 is electrically connected to the pixel electrode 47.
As described above, many of the liquid crystal display devices which are now widely used utilize the liquid crystal""s anisotropy of dielectric constant.
A liquid crystal display device using a magnetic field is proposed in Japanese Laid-open Publication No. 7-64118. The liquid crystal has also an anisotropy of magnetic susceptibility. This liquid crystal display device utilizes such an anisotropy of magnetic susceptibility. As shown in FIG. 18, this liquid crystal display device includes a pair of substrates 54 and a liquid crystal layer 55 interposed therebetween, wherein one of the pair of substrates 54 has a ferromagnetic element 53 including portions 53a and 53b. The region of the liquid crystal layer 55 which is interposed between the portions 53a and 53b is controlled by changing a magnetization of the ferromagnetic-element 53 by an external means 56 for applying a magnetic field.
A magnetic energy density fm of the liquid crystal molecules present in the magnetic field is generally given by the following expression:
fm=xe2x88x921/2"khgr"xe2x8axa5H2xe2x88x921/2xcex94"khgr"(nxc2x7H)2
where xcex94"khgr"="khgr"∥xe2x88x92"khgr"xe2x8axa5: anisotropy of magnetic susceptibility;
"khgr"∥: magnetic susceptibility in an alignment direction;
"khgr"xe2x8axa5: magnetic susceptibility in the direction perpendicular to the alignment direction; and
n: alignment direction of the liquid crystal molecules.
In the case where a magnetic field is applied to the liquid crystal molecules having a positive anisotropy xcex94"khgr", a moment is generated so that a magnetic energy is minimized. In other words, the liquid crystal molecules are aligned parallel to the direction of the magnetic field. In the case where a magnetic field is applied to the liquid crystal molecules having a negative anisotropy xcex94"khgr", a moment is generated so that a magnetic energy is minimized. In other words, the liquid crystal molecules are aligned perpendicular to the direction of the magnetic field.
It is understood from the foregoing that the alignment of the liquid crystal molecules can be controlled not only by the electric field but also by the magnetic field.
A conventional liquid crystal display device using an electric field utilizes thin film transistors for applying a signal voltage corresponding to a pixel to a corresponding pixel electrode.
Stable characteristics of the thin film transistors can be obtained by accurately aligning the respective patterns of the gate electrodes, semiconductor layer, source electrodes and drain electrodes with respect to each other. More specifically, a current flowing between the source and drain electrodes of each thin film transistor is proportional to a signal voltage applied to the source electrode, and substantially inversely-proportional to the distance between the source and drain electrodes. Moreover, a parasitic capacitance substantially proportional to the overlapping width of the gate electrode with each of the source and drain electrodes is produced in the thin film transistor. A potential at each pixel is determined by the current and parasitic capacitance as described above.
The distance between the source and drain electrodes is generally designed to about 10 xcexcm, and the overlapping width of the gate electrode with each of the source and drain electrodes is generally designed in the range of about 1 xcexcm to about 2 xcexcm. Furthermore, an accuracy of about 1 xcexcm or less is required with respect to the line width and the overlapping width. Therefore, highly accurate photolithography technology is conventionally used for the exposure step. In other words, the thin film transistors are produced using an high-performance exposure apparatus including a projection lens system, whereby the accuracy of about 1 xcexcm or less is satisfied.
Moreover, the semiconductor layer for the thin film transistors is generally formed from amorphous silicon (a-Si). In order to form a high-quality a-Si film, a PE-CVD (plasma enhanced chemical vapor deposition) apparatus must be used.
Such a liquid crystal display device has a high display quality. However, an expensive production apparatus is required because a highly accurate photolithography technology and a PE-CVD process are used. Moreover, such photolithography technology and PE-CVD process result in a poor production capability. Furthermore, the production process must be strictly managed.
According to the conventional example, charges based on the applied image signal are retained using the liquid crystal layer as a capacitor. Therefore, the liquid crystal layer must have a high specific resistance. However, in the case where the liquid crystal display device is driven at a high temperature of, for example, about 70xc2x0 C., the specific resistance of the liquid crystal layer may be disadvantageously reduced due to the ionic impurities produced within the liquid crystal layer, resulting in non-uniformity of the display. As a result, the production yield is reduced.
As can be seen from the foregoing, the conventional liquid crystal display device has difficulties in reducing the production cost and in improving the production yield.
The liquid crystal display devices are thin and light, as described above. However, for the above-mentioned reasons, the liquid crystal display devices are more expensive than the other image display devices such as a cathode ray tube (CRT). This is the main factor preventing the liquid crystal display devices from being used in a wider variety of apparatuses. Therefore, development of the liquid crystal display devices capable of being produced by a simple process has been desired.
Regarding the liquid crystal display device using a magnetic field, the above-cited Japanese Laid-open Publication No. 7-64118 describes the principle that the optical state of the liquid crystal layer can be changed by a magnetic field produced by a magnetic material. However, the actual driving method is not clearly described. In other words, Japanese Laid-open Publication No. 7-64118 fails to describe the liquid crystal display device having pixels arranged in a matrix, and a method for driving the same. More specifically, Japanese Laid-open Publication No. 7-64118 fails to describe a method for driving the liquid crystal display device which is used as an image display device in television sets, personal computers, word processors, OA equipments and the like. Accordingly, the liquid crystal display device described in Japanese Laid-open Publication No. 7-64118 can not be substituted for the conventional liquid crystal display device used as the image display device.
According to one aspect of the present invention, a liquid crystal display device includes: a liquid crystal layer; a plurality of row signal lines for driving the liquid crystal layer; and a plurality of column signal lines for driving the liquid crystal layer, wherein an optical state of the liquid crystal layer is varied by a magnetic field produced by at least one signal line of one of the plurality of row signal lines and the plurality of column signal lines.
In one example, at least one signal line of one of the plurality of row signal lines and the plurality of column signal lines is partially bent so as to have a signal line portion extending parallel to at least one signal line of the other of the plurality of row signal lines and the plurality of column signal lines, the signal line portion and the at least one signal line forming a parallel portion, and an optical state of the liquid crystal layer is varied by a magnetic field produced by at least one of the signal line portion and the at least one signal line which form the parallel portion.
In one example, at least one signal line of one of the plurality of row signal lines and the plurality of column signal lines is a first signal line, the first signal line being partially bent so as to have a signal line portion extending parallel to at least one other signal line of the other of the plurality of row signal lines and the plurality of column signal lines, the signal line portion and the at least one other signal line forming a parallel portion, the one of the plurality of row signal lines and the plurality of column signal lines includes a second signal line which is partially bent so as to overlap the first signal line, the second signal line and the first signal line interposing the liquid crystal layer therebetween, and an optical state of the liquid crystal layer is varied by a magnetic field produced by at least one of the second signal line, the signal line portion and the at least one other signal line.
In one example, a ferromagnetic element piece is provided adjacent to at least one of the signal line portion and the at least one signal line which form the parallel portion, and an optical state of the liquid crystal layer is varied by a magnetic field from the ferromagnetic element piece magnetized by the magnetic field produced by at least one of the signal line portion and the at least one signal line which form the parallel portion.
In one example, a ferromagnetic element piece is provided adjacent to at least one of the signal line portion and the at least one other signal line, and an optical state of the liquid crystal layer is varied by a magnetic field from the ferromagnetic element piece magnetized by the magnetic field produced by at least one of the signal line portion and the at least one other signal line.
In one example, a shielding electrode for preventing an electric field from being formed within pixel regions is formed in an inner periphery of each of the pixel regions, an optical state of each of the pixel regions being independently varied by a magnetic field from the plurality of row signal lines and the plurality of column signal lines.
In one example, an alignment direction of liquid crystal molecules in the liquid crystal layer is controlled by controlling a current flowing in the plurality of the row signal lines and the plurality of column signal lines.
According to another aspect of the present invention, a method for driving the liquid crystal display device includes the steps of: applying an image signal to the ferromagnetic element piece during a write period; and prior to the write period, sequentially applying a magnetic field for causing a magnetic field at the ferromagnetic element piece to have saturated magnetization, and an inverted magnetic field for eliminating magnetization of the ferromagnetic element piece, by using a magnetic field from at least one of the plurality of row signal lines and the plurality of column signal lines.
According to still another aspect of the present invention, a liquid crystal display device includes: a liquid crystal layer interposed between a pair of substrates; a plurality of first signal lines producing a magnetic field to be applied to the liquid crystal layer; and a plurality of second signal lines producing a magnetic field to be applied to the liquid crystal layer, wherein the liquid crystal layer includes a plurality of pixel regions arranged in a matrix, an optical state of each of the plurality of pixel regions is independently varied by the magnetic field produced by the plurality of first signal lines and the plurality of second signal lines, each of the plurality of first signal lines alternately has a plurality of first portions extending in a first direction and a plurality of second portions extending in a second direction perpendicular to the first direction, each of the plurality of second signal lines is provided in a vicinity of a respective one of the plurality of first portions and is respectively located between two other of the plurality of first portions which are adjacent to the respective one of the plurality of first portions, and an optical state of each of the plurality of pixel regions is varied by a magnetic field produced by the one of the plurality of first portions, the two other of the plurality of first portions, and a respective one of the plurality of second signal lines.
In one example, the liquid crystal display device further includes a third signal line formed on one of the substrates which f aces the other substrate having the first signal lines, the third signal line having a shape overlapping a shape of the first signal lines, wherein an optical state of each of the plurality of pixel regions is varied by a magnetic field produced by the one of the plurality of first portions, the two other of the plurality of first portions, a respective one of the plurality of second signal lines, and the third signal line.
In one example, the liquid crystal display device further includes a ferromagnetic element piece provided adjacent to the plurality of first portions of the first signal lines, wherein an optical state of each of the plurality of pixel regions is varied by a magnetic field from the ferromagnetic element piece magnetized by the one of the plurality of first portions, the two other of the plurality of first portions, a respective one of the plurality of second signal lines, and the third signal line.
According to a liquid crystal display device of the present invention, an optical state of a liquid crystal layer is varied by a magnetic field produced by at least one of row signal lines and column signal lines. Therefore, an image can be displayed by, for example, pixels arranged in a matrix, whereby non-uniform display depending upon the specific resistance of the liquid crystal layer will not be produced.
At least one signal line of the row signal lines and/or the column signal lines is bent so as to have a portion extending parallel to a corresponding column signal line, whereby the portion and the corresponding column signal line form a parallel portion. With such a structure, the liquid crystal display device can be driven on a pixel-by-pixel basis. Moreover, the liquid crystal layer can be driven by using the structure and production process which do not require a high pattern accuracy. As a result, an excellent display quality can be obtained.
A ferromagnetic element piece is formed adjacent to at least one signal line of the row signal lines and/or the column signal lines. The ferromagnetic element is magnetized by a magnetic field produced by at least one of a corresponding row signal line and/or a corresponding column signal line, and a magnetic field to be applied to a pixel region is produced by the magnetized ferromagnetic element piece. Therefore, a magnetic field to be applied to the pixel region can be increased.
A shielding electrode for preventing an electric field from being formed within a pixel region by a magnetic field from the row signal lines and the column signal lines is formed in a periphery of each pixel region. Accordingly, a voltage across the row signal lines and the column signal lines can be increased, whereby a sufficient magnetic field can be produced by the row signal lines and the column signal lines.
An alignment direction of the liquid crystal in the pixel regions is controlled by controlling a current across the row signal lines and the column signal lines.
Therefore, a magnetic field can be arbitrarily controlled.
At least one of the row signal lines, the column signal lines, the shielding electrodes and insulating layers provided therebetween, or at least one of the row signal lines, the column signal lines, the shielding electrodes, the ferromagnetic element pieces and insulating layers provided therebetween is formed by a printing method. Therefore, expensive apparatuses having a poor production capability, such as a vacuum film-forming apparatus and an exposure apparatus, are not necessary. As a result, significant reduction in production cost can be achieved.
At least one of the insulating layers provided between the row signal lines, the column signal lines and the shielding electrodes, or at least one of the insulating layers provided between the row signal lines, the column signal lines, the shielding electrodes and the ferromagnetic element pieces is formed by a coating method. Therefore, an expensive vacuum film-forming apparatus having a poor production capability is not necessary. As a result, significant reduction in production cost can be achieved.
According to a method for driving a liquid crystal display device of the present invention, (i) a magnetic field exceeding saturated magnetization of the ferromagnetic element pieces and (ii) an inverted magnetic field which eliminates magnetization of the ferromagnetic element pieces are sequentially applied to at least one of the row signal lines and the column signal lines for a prescribed time period prior to a write period for applying an image signal to the ferromagnetic element pieces. Therefore, magnetization of the ferromagnetic element pieces can be arbitrarily controlled to a level corresponding to the applied image signal.
Thus, the invention described herein makes possible the advantages of (1) providing an inexpensive liquid crystal display device capable of achieving both reduction in production cost and improvement in production yield, and (2) providing a method for driving the same.